In electrophotographic printing, a pattern of electrostatic charges corresponding to a print image is developed on an optical photoreceptor (OPR). Toner is applied to the OPR and that toner which is retained as a result of not being repelled by electrostatic charges is used to form the print image. The print image is then transferred to a print media (usually paper).
The OPR may work with either visible spectrum light or optical energy outside the visible light spectrum. In the preferred embodiment, it is anticipated that near infrared laser light will be used, but the OPR as described in connection with this invention is intended to mean any photoreceptor which responds to radiated energy.
A laser printer creates a printed image by causing a laser light source, such as a laser diode, to scan across the charged surface of photosensitive material on an optical photoreceptor (OPR) in a succession of scan lines. Each scan line is divided into pixel areas and the laser beam is modulated such that selected pixel areas are exposed to light. The exposure to light results in the depletion of surface charges. The exposure of the OPR to the light thereby discharges the OPR at that location and results in the OPR developing toner and then transferring the toner to a corresponding location on the print media (usually a sheet of paper).
It is not practical to modulate the power of the laser other than to turn it on and off. There are too many environmental factors that are difficult to control that make operation of the diode in a non-saturated mode impractical. We can accomplish the same effect however by turning the laser (full) on and off for periods of time shorter than the actual pixel size. This is a standard application of a pulse-width modulated signal. In addition, there are times (to prevent jagged edges in a diagonal line, for example) when we wish to skew the pixel portion to the right or left, as well as change its effective intensity. It is therefore desired to provide an laser printer which allows the laser to be turned on and off in a precisely controlled manner. It is also desired to provide a design that permits this type of modulation that is phase locked to the scanning of the laser beam; otherwise successive lines will not line up accurately.
At locations where the OPR charge is depleted (by the laser light), toner particles from the emulsion are concentrated, thereby creating the image. At locations on the OPR which are charged, toner particles are not retained by the OPR (the non-image area). This makes the laser printer particularly adaptable to a rasterized print pattern, although it is possible to configure a laser printer for other types of scan techniques.
Printer resolution is partially a function of the size of the optical image which is generated by the laser or optics. The present invention is directed to enhancements of the optical image.
Hewlett-Packard Company, the assignee of this patent, has developed a technique for enhancing images for hardcopy devices which produce pixelated images. The technique includes matching a bit map of an image to be printed with predetermined stored templates or patterns to detect occurrence of preselected bit map features. When a match occurs, a compensated pattern is generated, which results in print enhancement. This technique is described in U.S. Pat. No. 4,847,641, to Charles Chen-Yuan Tung, and commonly assigned. One result of the technique is an ability to change the size of pixels along the edges of diagonal lines in order to reduce the jagged edges of these lines. In present printer configurations, more sophisticated algorithms are used to improve resolution.
Laser printers print pages by applying black toner to selected small regions of fixed size called pixels. It is possible to create the effect of shades of gray by placing toner in only a portion of the pixel region. This can be done by using pulse width modulation (PWM). It is not sufficient just to provide pulse width modulation of the pixel. It is also necessary to lock the phase of the pixels with the start of the laser scan; a single pulse synchronization signal from the laser scanner called "beam detect" provides the scan phase information.
There are several ways to vary the optical energy levels within a pixel. One common technique, pulse width modulation (PWM), is to simply vary the time duration of the energy applied to the optical element (the laser) within the pixel scan time. In this manner, the energy level is established by the time that the laser is turned on, rather than the power output of the laser.
A phase locked loop as known in the prior art cannot be used because in a laser printer there is no feedback or error signal that can correct frequency phase drift. The beam detect signal provides a single edge on each scan, so is of such low frequency that it cannot be used to establish a phase lock loop. The clock generator described in U.S. Pat. No. 5,438,353, commonly assigned, provides a description of the phase locking which may be used for this invention. The clock generator generates a clock signal that is exactly phase aligned to the beam detect signal transition, within the limits of a quantizing effect due to an internal delay chain. It does this alignment once, and does not change the phase again until the next beam detect signal occurs. The beam detect signal is generated by the printer engine as the laser begins a scan, which necessarily occurs once per line.
The aforementioned U.S. Pat. No. 5,438,353 describes controlling the timing of a modulation input signal utilized for driving a laser driver circuit. The present invention is an improvement on the capability of the clock generator describe in U.S. Pat. No. 5,438,353. It is no longer considered sufficient to print a pixel "ON" or "OFF." It is considered desirable to be able to provide continuously variable pixel outputs.
Laser printers are distinguished from other printers by their ability to place precise amounts of toner into very small regions of the page at relatively high speed, thus resulting in image quality far greater than most other types of printers. Since laser printers operate by scanning a drum or other optical photoreceptor that holds the printed image, this results in an intrinsic quantization of the image in the vertical direction of the page as the page passes the optical photoreceptor. In addition, limitations in the circuitry that modulates the horizontal scanning also quantizes the image, so that a single cell or pixel is effectively formed. If pixels are made small enough, the quantization effects can be made too small to see. There are, however, practical limits; the vertical quantization is limited by the ability to transfer data in serial form to the scanning laser.
Laser printer resolution is currently sufficient that the quantization is not normally visible, even at the pixel level. There are several indirect effects of quantization that produce interference effects that are generally called artifacts. Such flaws are readily apparent when the laser printer is used to reproduce photographic images. This is because of the extensive use of halftones in photographic image reproduction. Artifacts often appear as a printer attempts to print continuously varying halftones shades with quantized regions of pigmented and nonpigmented areas. By mixing regions of black and white (in the case of black pigment), and yet keeping the regions extremely small it is possible to provide an image which appears to the eye as a continuous range of gray. If the laser output can be modulated into very small regions of charged and uncharged areas, smaller than the quantized pixel elements, it is possible to reduce artifacts in printed photographs.
This problem has been addressed by providing several clocks with varying phase. In one such approach, the clock phases are locked the laser scanning via a beam detect signal, indicating the start of a scan line. It is possible to thereby provide precise pulse width modulation of up to 1/8 pixel, even at very high horizontal pixel rates. Advances in printer engine technology and speed as well as the desire to print high quality photographic images require even finer pulse width modulation steps.
A prior art technique uses a capacitor with a current source and a voltage comparator. A change in voltage resulting from the current applied to the capacitor is supplied to the voltage comparator, so that the circuit will trigger on and off at controlled times. This system has potential drawbacks in that the re-initialization time limits the maximum operating frequency and the circuit presents linearity problems at either end of a pulse width modulation range (0% and 100%). It also has problems with stability over temperature and voltage and die lot (component) variation. A very significant problem is that this technique is limited in frequency. It is expected that printers will continue to increase in scan and print speeds, and for that reason, the capacitor method is not advantageous.